1. Field of the Invention
The present invention relates to a method of correcting the balance for a high-speed rotatable body, for instance, a method of correcting the balance for a rotatable body requiring high-speed rotation applied to an air bearing polygon scanner. The present invention further relates to a dynamic pressure bearing and an optical scanning apparatus using the dynamic pressure bearing.
2. Discussion of the Background
A balance correction method for correcting unbalance of a rotatable body has been well known hitherto. As to such methods for balance correction, the following methods (1), (2) described below are known.
(1) One balance correction method rotates the rotatable body and respectively detects the vibration occurring at the time of rotating the rotatable body and the standard position of the rotatable body, and additionally provides the weight on the desired position of the rotatable body in order to cancel the unbalanced mass causing the vibration.
(2) Another balance correction method removes a part of the rotatable body constructing part.
In recent years, in order to realize high-speed printing and high resolution of an image (high image quality) in a digital copying machine, etc., it has become necessary to rotate a polygon scanner with super-high speed more than 30,000 rpm and with high precision. However, when the rotatable body is rotated with high speed, the problem of vibration due to the unbalance of the rotatable body becomes serious, and therefore balance correction may become necessary in order to correct the unbalance of the rotatable body.
When a semiconductor laser (e.g., laser diode) is employed as the light source, an optical scanning apparatus in which the laser beam emitted from the light source is deflected and scanned by use of the rotating polygon mirror serving as the polarizing unit is used in the laser printer or the copying machine which is one type of image forming apparatus.
For the rotatable polygon mirror serving as the polarizing unit, a metal mirror having plural reflection surfaces formed by directly grinding aluminum is generally used. The metal mirror is rotatively driven by a motor for driving the mirror, and thereby the mirror can deflect the laser beam emitted from the light source in order to perform the scanning operation.
In recent years, as to the bearing structure for the driving motor, an air bearing structure referred to as a xe2x80x9cdynamic bearingxe2x80x9d has been employed.
Contrary to a ball bearing with a rotation speed equal to or lower than 10,000 rpm, the dynamic pressure bearing rotates with a high rotation speed (revolutions number) equal to or higher than 30,000 rpm. At the time of the rotation, the dynamic pressure bearing takes the surrounding air thereinto and supports the rotor in the radial direction in a floating state, and further the bearing causes the rotor to rotate without being brought into contact with the surrounding portions in the floating state of the rotor therefrom in the up-and-down direction by utilizing the magnetic force of the magnet. In the structure of such a dynamic pressure bearing, the rotor is not brought into mechanical contact with the surrounding portion. Furthermore, the structure does not need to pour any lubricating agent (e.g., oil), and thereby a maintenance-free bearing can be realized.
As to the structure of a dynamic pressure bearing employed for a rotatable polygon mirror, the published specifications of Japanese Laid-open Patent Publication No. 6-110007, Japanese No. 2707453, and Japanese Laid-open Patent Publication No. 7-190047 respectively disclose structures of dynamic pressure bearings.
The published specification of the Japanese Laid-open Patent Publication No. 6-110007 discloses a unitary integrated structure of a rotatable shaft fitted to the fixed shaft provided in the main body of the machine as a rotatable body, a pedestal mounted on the outer circumference of the rotatable shaft, a polygon mirror and magnet respectively fitted and firmly combined, and such that the pedestal is brought into direct contact with the lower surface of the rotatable polygon mirror. Thereby, the horizontality rate of the rotating surface is kept and is constructed with aluminum employed as a light and low-inertia substance, and the pedestal and the rotatable polygon mirror are bonded to the rotatable shaft.
The published specification of Japanese Patent No. 2707453 discloses another structure in which plural rotatable polygon mirrors for deflecting the laser beam are prepared and when the diameter of the connecting portion for connecting respective polygon mirrors in the axis direction thereof is larger than the diameter of a spring washer for holding the polygon mirrors by pushing them down from above, and the polygon mirrors are respectively concentric with an inscribed circle of the rotatable polygon mirrors, the connecting portion is formed such that the diameter of the portion is smaller than that of the inscribed circle, and the rotatable polygon mirror can be prevented from being distorted or deformed due to the pressure at the time of pressing the mirrors by use of the spring washer.
The published specification of Japanese Laid-open Patent Publication No. 7-190047 discloses still another structure in which, for a high-speed rotatable body provided with an outer circumferential member made by a firmly attached (e.g., bonded) metal, by a shrinkage fitting method to a ceramics sleeve and an outer circumference thereof, the inner side (or wall) of the ceramic sleeve is formed in the shape of a drum after firmly attaching (e.g., bonding) the ceramics sleeve to the metal outer circumferential member utilizing the above-mentioned shrinkage fitting method. The inner side of the ceramics sleeve is formed in the predetermined drum shape such that the gap is formed between the outer circumferential surface of the fixed shaft made of ceramics and the inner circumferential surface of the ceramics sleeve to be tightly fittedly inserted thereinto, in accordance with the shrinkage fitting compression stress eased by the thermal expansion due to the centrifugal force and the frictional force at the time of the high-speed rotation, and the drum shape is made equal to the compression deformation corresponding to the shrinkage fitting compression stress caused by the temperature rising-up at the time of using the bearing. Thereby, the initial gap of the bearing can be kept uniform.
Heretofore, the background arts regarding a balance correcting method for a high-speed rotatable body, a dynamic pressure bearing, and an optical scanning apparatus utilizing the dynamic pressure bearing have been described. However, according to such background arts, for instance as disclosed in the noted background-art documents, e.g., the documents describing the balance correcting method, and the published specifications of Japanese Laid-open Patent Publication Nos. 6-110007 and 7-190047 and Japanese Patent No. 2707453, etc., there is not provided any advantageous functional effects for improving the balance correcting method, the dynamic pressure bearing, and the optical scanning apparatus.
The present invention has been made in view of the above-discussed and other problems and one object of the present invention is to address the above-mentioned defects and troublesome matters of the background arts.
In more detail, since the greater the revolutions number at the time of correcting balance, the more improved the vibration detecting ability, the rotatable body is desired to be rotated with high speed. However, a method of adding weight causes a troublesome matter for the balance correction due to the high-speed revolutions, because the weight is scattered by the action of centrifugal force.
Furthermore, in a method of removing a part of the rotatable body constructing parts, the above-mentioned troublesome matters do not occur at all. However, immediately after assembling the rotatable body before correcting the balance, an unbalance due to unevenness of the respective construction parts turns out to be large. On such a condition, when the rotatable body is rotated with high speed, the amplitude of the vibration becomes very large. As the result, the bearing may be considerably damaged. To state more concretely, when a rolling bearing such as a ball bearing, etc. is employed, the ball surface and the inner and outer rings may be damaged. When a fluid bearing such as a dynamic pressure bearing, etc. is employed, since the load capacitance thereof is small, the bearing may be brought into contact with other parts. As the result, the bearing may also be damaged.
The above problems can be reduced by decreasing the initial unbalance. However, in a case that the revolutions number is equal to or larger than 30,000 rpm, the accuracy of the mechanical processing for the fitting portions of the respective construction parts has to be on the order of not larger than several xcexcm, on all occasions. Consequently, the cost of assembling the parts of the bearing may increase, and further temperature control may be necessary at the time of assembling the bearing. As a result, there arise many other problems to be solved.
To address the drawbacks in the background art noted above, according to the present invention, a method of attaching a weight onto a desired position of the rotatable body and another method of removing a part of the rotatable body constructing parts, both in the process of correcting the balance, are jointly utilized. Thereby, the accuracy of correcting the balance can be improved, and thereby a polygon scanner with a the low vibration can be provided.
Next, subject matter to be solved regarding the dynamic pressure bearing and the optical scanning apparatus using the dynamic pressure bearing is discussed.
In the structure of the dynamic pressure bearing employed in the rotatable polygon mirror, as disclosed in the published specification of the Japanese Laid-open Patent Publication No. 7-190047, the temperature of the construction member rises due to thermal friction between the rotatable body side and air occurring at a time of the rotation. For this reason, when there exists a difference in the thermal expansion amount between the structural members, a looseness occurs in the junction parts thereof and the balance of the rotatable body is deteriorated.
Regarding the above-mentioned matters, the published specification of Japanese Laid-open Patent Publication No. 7-190047 discloses mitigation of the shrinkage fit compression stress caused by the thermal expansion. The unevenness of the gap formed between the outer circumferential surface of the fixed shaft made by ceramics and the inner circumferential surface of the ceramics sleeve fittedly inserted into each other is prevented as a result of the shape of the inner surface of the ceramics sleeve. None of the other published specifications disclose the above-mentioned points. In particular, in the published specifications of Japanese Laid-open Patent Publication No. 6-110007 and Japanese Patent No. 2707453, there is disclosed a low-inertia structure or a structure only aiming at decreasing the thickness of the rotatable polygon mirror, with the intention of reducing the load of the driving source at the time of starting the rotation.
If there is a difference between the thermal expansion amounts of the respective structural members at the connecting portions thereof, the connecting relationship between the rotatable polygon mirror as one of the rotatable bodies and the sleeve fittedly fixed on the outer circumferential surface of the polygon mirror deviates from the normal state. As a result, a surface falling-down may occur on the reflection surface of the rotatable polygon mirror, or the position of the rotational gravity center may change at the time of rotation, and thereby the balance condition worsens. Consequently, the above matters are a cause of vibration.
Regarding the vibration at the time of rotation, the published specification of Japanese Patent No. 2707453 discloses provision of a weight mounting portion for taking a moving balance on an upper surface of a connecting portion. However, such a structure is only employed when the rotatable polygon mirror is mounted on the rotatable shaft. The structure is not one in which the bearing on the rotatable shaft is made in the dynamic bearing structure. Namely, the above background-art document does not disclose any thing aiming at improvement of the thermal expansion on the dynamic pressure bearing.
On the other hand, with respect to the respective published specifications regarding the thermal expansion, for instance, the Japanese Laid-open Patent Publication No. 6-110007 discloses a structure in which parts around the rotatable body are fixed thereon with adhesive agent or are pressingly fixed thereon by use of a plate spring. In such a structure, the thermal stress is apt to occur very frequently due to the temperature rising-up at the adhesive agent portion or the plate spring and the centrifugal force caused by the rotation. Consequently, the rotation balance may become unstable.
In the structure of Japanese Patent No. 2707453, the connecting portion and the rotatable polygon mirror are pressingly fixed to each other by a plate spring, and thereby when the thermal stress occurs the pressing fixture relationship deviates from the normal state and the rotation balance may become unbalanced.
In the structure disclosed in Japanese Laid-open Patent Publication No. 7-190047, as mentioned above, the rotation balance is corrected to some extent. However, since the thermal expansion of the outer circumferential member made by the metal is larger than that of the ceramics sleeve and the ceramics sleeve is firmly fixed directly to the metal-made outer circumferential member, and even though a drum-shaped member having the circumferential surface without any ceramics sleeve is provided with a shape changing deformation corresponding to the thermal stress, the occurrences of a thermal expansion difference between both of the above members cannot be avoided. Consequently, it is impossible to prevent changing of the reflection surface of the rotatable polygon mirror.
The present invention solves the subject matters mentioned heretofore and provides an improved balance correcting method for a high-speed rotatable body, such as an improved dynamic pressure bearing, and an improved optical scanning apparatus by utilizing the above-mentioned dynamic pressure bearing.
An object of the present invention is to provide a novel method of attaching a weight onto a desired position of a rotatable body and of removing a part of the rotatable body constructing parts, both jointly utilized in a process for correcting balance. In such a method as mentioned above, the accuracy of correcting the balance can be improved, and thereby a polygon scanner of low vibration can be provided.
Another object of the present invention is, in consideration of the subject matters to be solved regarding the above-mentioned background-art dynamic pressure bearing and an optical scanning apparatus utilizing the above dynamic pressure bearing, to provide a novel dynamic pressure bearing capable of stabilizing rotation balance and an optical scanning apparatus provided with a structure capable of not changing a direction of a reflection surface of a rotatable polygon mirror.